This script will process an image, take the colour used at each pixel and list them in order of use

Reading the Colours From an Image Using PHP

function colorPalette($imageFile, $granularity = 5){
	$granularity = max(1, abs((int)$granularity));
	$colors = array();
	$size = @getimagesize($imageFile);
	if($size === false){
		user_error("Unable to get image size data");
		return false;
	}
//$img = @imagecreatefromjpeg($imageFile);

if ($size[2]==1)
$img = @imagecreatefromgif($imageFile);
if ($size[2]==2)
$img = @imagecreatefromjpeg($imageFile);
if ($size[2]==3)
$img = @imagecreatefrompng($imageFile);

if(!$img)
{
user_error("Unable to open image file");
return false;
}
for($x = 0; $x < $size[0]; $x += $granularity)
{
for($y = 0; $y < $size[1]; $y += $granularity)
{
$thisColor = imagecolorat($img, $x, $y);
$rgb = imagecolorsforindex($img, $thisColor);
$red = round(round(($rgb['red'] / 0×33)) * 0×33);
$green = round(round(($rgb['green'] / 0×33)) * 0×33);
$blue = round(round(($rgb['blue'] / 0×33)) * 0×33);
$thisRGB = sprintf('%02X%02X%02X', $red, $green, $blue);
if(array_key_exists($thisRGB, $colors))
{
$colors[$thisRGB]++;
}
else
{
$colors[$thisRGB] = 1;
}
}
}
arsort($colors);
return $colors;
}
function getHtml2Rgb($str_color)
{
if ($str_color[0] == '#')
$str_color = substr($str_color, 1);

if (strlen($str_color) == 6)
list($r, $g, $b) = array($str_color[0].$str_color[1],
$str_color[2].$str_color[3],
$str_color[4].$str_color[5]);
elseif (strlen($str_color) == 3)
list($r, $g, $b) = array($str_color[0].$str_color[0], $str_color[1].$str_color[1], $str_color[2].$str_color[2]);
else
return false;

$r = hexdec($r); $g = hexdec($g); $b = hexdec($b);
$arr_rgb = '('.$r.','. $g.','. $b.')';
// Return colors format liek R(255) G(255) B(255)
return $arr_rgb;
}
function totalpx($imageFile, $granularity)
{
$size = @getimagesize($imageFile);
$totpx = $size[0]*$size[1];
$pcrpx = round($totpx / $granularity);
return $pcrpx;
}

// sample usage:
$images = 'http://media.gizmodo.co.uk/wp-content/uploads/2012/08/virgin-trains-franchise.jpg';
$granularity = 4;
$palette = colorPalette($images, $granularity);
$total_pixel = totalpx($images, $granularity);
$colors_to_show = 255;

echo '<img src="'.$images.'"><br/>';
echo '<table border="1"><tr><td>Color</td><td>Color Hex</td><td>Color RGB</td><td>Count</td><td>Percentage</td></tr>';
for($h=0;$h<$colors_to_show;$h++) {
$color = array_keys($palette);
$hex = '#'.$color[$h];
$color_pixel = $palette[$color[$h]];
$percentage = ($color_pixel / $total_pixel) * 100;
echo '<tr><td style="background-color:'.$hex.';width:2em;"> </td><td>'.$hex.'</td><td>rgb'.getHtml2Rgb($hex).'</td><td>'.$palette[$color[$h]].'</td><td>'.number_format($percentage, 1).' %</td>';
}
echo '</table>';